This invention relates to an electronically controllable tuning device with a frequency synthesizer which controls a local oscillation frequency by a phase locked loop including a programmable frequency divider having a frequency division ratio controlled by a channel selection device.
Known is an electronically controllable tuning device with a frequency synthesizer which includes a phase locked loop having a programmable divider for frequency dividing the oscillation frequency of a local oscillator, means for varying the frequency division ratio of the programmable divider according to the frequency of a selected broadcasting electromagnetic wave and means for feeding back to the local oscillator the output of a phase detector for comparing phases of a reference frequency and the output of a programmable divider to control the oscillation frequency of the local oscillator and supplies the controlled oscillation frequency as a local oscillation signal to an intermediate frequency generating mixer. The outline of the electronically controllable tuning device with a frequency synthesizer is disclosed in a magazine "NIKKEI ELECTRONICS" pages 59 to 61, published on July 29, 1974 by NIKKEI MacGraw-Hill. Where, however, such a known electronically controllable tuning device is applied to a television tuner for use in an ultra-high frequency (UHF) band, a frequency division circuit including the above-mentioned programmable frequency divider must be operated at a high frequency of about 1 GHz. This is due to the fact that in Japan, for example, the oscillation frequency of the local oscillator in the UHF television tuner is determined to be in a range of 530 to 824 MHz. Such a frequency division circuit is difficult to manufacture and operate in a stable way and therefore unsuitable from a practical viewpoint.
It is accordingly the object of this invention to provide an electronically controllable tuning device with a frequency synthesizer which can be suitably put to practice by making a frequency treated in a phase locked loop lower than a local oscillation frequency which is supplied to a mixer.